Optical interferometry has been developed as a technique by which to improve resolution of stellar imaging. Several observatories in the world are equipped to perform optical interferometry. These observatories have been designed primarily for stellar observations. Within the United States, observatories include the Navy Prototype Optical Interferometer (NPOI), the Center for High Angular Resolution Astronomy (CHARA), and the Magdalena Ridge Observatory Interferometer (MROI). In addition, there are two optical interferometer observatories in other parts of the world, including Very Large Telescope Interferometer (VLTI) in Chile and Sydney University Stellar Interferometer (SUSI) in Australia.
The CHARA telescope array, for instance, is laid out in a Y-pattern with two telescopes per arm, with each arm being about 200 meters in length. Pair-wise combinations of the telescopes permit fifteen different baselines ranging in length from about 30 meters to about 300 meters, with several baselines having similar lengths but lying in different orientations. From each telescope, a 6 inch diameter collimated beam is propagated through an evacuated pipe back along the arm of the Y to a 94 meter building at the vertex that encloses about 1,000 square meters of optical laboratory floor space. A majority of this space is occupied by six optical delay lines. The space also contains beam combiners, cameras, and other equipment.
Each delay line includes a series combination of a discretely-selectable segment, which has five possible settings ranging from 0 meters to approximately 146 meters, and a continuously-variable segment including a cat's-eye retro-reflector mounted on a cart that moves on rails 46 meters long. A total delay ranging from 0 meters to approximately 146 meters is therefore possible. The long continuously-variable segment of the optical delay is needed to interferometrically image stars as the stars move across the sky. The beam transport pipes and the optical delay lines are major cost and facilities drivers in existing laboratories.
Many high-value commercial and military satellites orbit the Earth in geosynchronous Earth orbit (GEO) at an altitude of 34,800 kilometers. There may be economic and national security motivations for wanting to obtain resolved images of geosynchronous satellites (geosats) from the ground. A typical geosat body, called a bus, may have a maximum dimension of 2 to 10 meters. Antennas and solar panel widths may be of similar size. Solar panel lengths, for example, may be from 10 to 100 meters. However, even with the largest telescopes available, which may have apertures of 8 to 10 meters and may be equipped with adaptive optics to compensate atmospheric turbulence, only the largest features of geosats may barely be resolvable. Indeed, only a few of the largest stars may be resolvable.